1. Field of the Invention
This invention relates to an image forming device such as a duplicator, a printer, a facsimile or the like in which a printing operation is carried out utilizing an electrophotographic recording system and an electrostatic recording system.
2. Description of the Related Art
In this kind of image forming device, a plurality of devices such as a photosensitive member, a developing unit, a transfer unit, a fixing unit or the like are provided as working parts, but obviously the life-time of these working parts is generally shorter than that of durable parts such as a motor or the like. Accordingly, these working parts having a relatively shorter lifetime are currently handled as consumable goods and are exchanged when their life-time is exhausted. To this end, in current maintenance procedures for a large-size image forming device, these consumable working parts are examined by an experienced operator at suitable intervals, and are exchanged at the discretion of the operator. In the maintenance procedure of a small-size image forming device, however, these consumable working parts are generally examined by the user, and therefore, such a small-sized image forming device is usually provided with a counter and a memory, the number of sheets printed by the image forming device is counted and integrated by this counter, and the integrated number of sheets printed in this image forming device is stored in the memory. In one type of these image forming devices, these consumable working parts are exchanged when a predetermined total number of printed sheets or number of sheets integrated by the mechanical counter as explained above is reached, or upon a lessening of the quality of the printed characters. Therefore, when the total number of sheets printed in the image forming device reaches the predetermined number previously set for a certain consumable working part of this device, the requirement for an exchange of that certain consumable working part will be generated and displayed to the user.
Usually, however, a simple proportional relationship between the lifetime of those consumable working parts and the number of printed sheets does not exist, and further, the lifetime of these consumable working parts is greatly affected by the conditions of the current usage of the image forming device, and thus it is difficult for the user to determine the correct intervals for an exchange of these consumable working parts.
For example, the photosensitive characteristic of a photosensitive member used in an image forming device utilizing an electrophotographic recording system will be deteriorated by a repeated incidence of a corona radiation from a precharging unit, a writing operation, an incidence of a corona radiation from a transfer unit and an incidence of a light from a lamp in a discharge unit, and further deteriorated by wear stemming from repeated contact with a developing substance, sheets to be printed, and a cleaner. This deterioration will occur not only in a period T.sub.p of a printing operation but also in a period T.sub.PRE of a process carried out prior to the printing operation (in which an initializing of the process, a charging, a writing, a sheet supply or the like is carried out.) and in a period T.sub.aft of a process carried out after the printing operation is completed (in which a discharge of the photosensitive member, a cleaning thereof a feeding of sheets out of the device or the like is carried out.).
The extent of such deterioration occurring during the printing operation, and caused mainly by the deterioration of, for example, the photosensitive member, will vary greatly between an image forming device in which the printing operation is repeatedly carried out every time one cut sheet is printed, as shown in FIG. 7(A), and an image forming device in which a continuous printing operation is carried out as shown in FIG. 7(B).
As a result, in the case shown in FIG. 7(A), often a consumable working part will be used even after the lifetime thereof is exhausted, and in the case shown in FIG. 7(B), a consumable working part must be exchanged before the lifetime thereof is exhausted.